The invention relates to a process which can be carried out continuously for preparing impact-modified styrene polymers (HIPS) which comprise, as disperse phase in a polystyrene matrix, a styrene-butadiene block copolymer or a particulate polybutadiene rubber compatibilized by grafting with styrene, by anionic polymerization of styrene in a polymerization reactor, or in more than one polymerization reactor arranged in sequence, in the presence of a rubber prepared in an immediately prior process, and preferably in the presence of a subordinate amount of an alkylaromatic solvent other than styrene, in particular toluene, ethylbenzene or mixtures of toluene and ethylbenzene.
Impact-modified polystyrene is usually prepared by bulk free-radical polymerization, where the rubber, obtained in a separate process, is dissolved in the styrene. In addition, use is usually made of a small amount of a solvent, such as ethylbenzene. The polymerization is carried out industrially in more than one reactor arranged in sequence, known as a cascade. Processes of this type have been reviewed, for example, by Echte in Handbuch der Technischen Polymerchemie, Weinheim, 1993; specific arrangements are described, for example, in U.S. Pat. Nos. 2,727,884 and 3,903,202.
It has also been proposed that impact-modified polystyrene may be prepared by anionic polymerization. This could be done, for example, in tubular reactors, the heat of polymerization being dissipated through the wall. To this end static mixers are incorporated in the cross-section of the tube, and encourage thorough radial mixing of the polymer (Nguyen Khac Tien et al. Chem. Eng. Technol. 13, 214-220 1990). However, no process of this type has become well established.
Rubbers, such as polybutadiene, are usually prepared in solution. Reference may be made in this connection to the review in Encyclopedia of Polymer Science and Technology Vol. 2, N.Y., 1985.
Anionic polymerization, as used for preparing block rubbers (styrene-butadiene block copolymers), for example, always requires a solvent (EP-A-510 410; EP-A-554 142; U.S. Pat. No. 5,286,457).
The disadvantage of known processes for preparing impact-modified polystyrene is that if a rubber is to be used for impact-modification it has firstly to be isolated from its reaction mixture (i.e. from a solution whose strength is, for technical reasons, at most from 15 to 20%).
Attempts have already been made to find industrially useful processes which dispense with the costly isolation of the rubber.
For example, European Patent Application 334 715 proposes a process for free-radical-initiated styrene polymerization in which polybutadiene is firstly prepared by anionic polymerization in ethylbenzene in a stirred reactor. Following termination of the reaction, preheated styrene is to be added as diluent, followed by polymerization. However, even this process would not be cost-effective in operation because of the amount of (high-boiling) solvent which is necessary for preparing the rubber and which has to amount to from 40 to 100% by weight of the amount of styrene subsequently added.
The British Patent 1 013 205 therefore proposes that butadiene is initially polymerized in a relatively low-boiling solvent, specifically cyclohexane, and that the solution is mixed with styrene and fractionated by distilling off the cyclohexane together with any remaining butadiene monomer. The solution of the rubber in styrene thus obtained is then to be used in the manner which is usual in styrene polymerization. This per se elegant process has not been introduced industrially, probably because, as indicated in the British Patent, the cyclohexane would have to be removed in an additional step to allow the polymerization to be carried out with an adequately high concentration of monomer.
These considerations also apply of course to anionic polymerization processes for preparing impact-modified polystyrene. On the other hand, in continuous anionic polymerizations it would scarcely be possible to use a rubber which had been isolated from its solution, because of the auxiliaries added during isolation. This is probably one of the reasons that the preparation of impact-modified polystyrene by anionic polymerization has not been introduced on an industrial scale, although it has the advantage per se of giving virtually monomer-free polystyrene.
European Patent 059 231 clearly describes the disadvantages of the process of the British Patent 1 013 205. A process is then proposed in which styrene and butadiene are initially processed anionically in a stirred reactor, i.e. with back-mixing, to give a styrene-butadiene block copolymer. The reaction is conducted in such a way that the reaction mixture finally obtained contains butadiene. The living chains are treated with a terminating (quenching) agent, and the excess of monomeric butadiene is then removed. Styrene is then added, followed by free-radical polymerization. Even this process is not cost-effective, since it requires the removal of excess butadiene in a separate step, so as not to impair the styrene polymerization.
In summary, it can be said that all of the processes described hitherto for preparing impact-modified styrene either have additional steps for removing solvent or residual monomers from the rubber synthesis or are dependent on prior isolation of the rubber. Overall, purely anionic preparation of impact-modified styrene on an industrial scale is a problem which remains unsolved.
It is an object of the present invention to put forward a cost-effective process for the purely anionic preparation of impact-modified styrene (HIPS) which avoids the disadvantages of known processes and does not require intermediate isolation of the rubber from its solution. A further object is to put forward an anionic polymerization process which to a substantial extent can be utilized in plants which already exist for the free-radical preparation of impact-modified polystyrene.
We have found that this object is achieved by means of a process which has at least one system of circulating solvent and, simply expressed, the significant feature that the rubber required is introduced to the styrene polymerization plant in the form of a solution in a third solvent which is neither styrene nor ethyl-benzene and whose boiling point is lower than that of styrene, and that of ethylbenzene if present, and that at least some of this third solvent is reintroduced after being removed from at least one of the polymerization reactors, by means of distillation. It is advantageous here for at least some of the heat of vaporization required as a result of evaporative cooling to be supplied via the heat of polymerization.
A second system of circulating solvent results from the devolatilizing, in a manner known per se, of the polymer formed, i.e. the impact-modified polystyrene, i.e. from the removal of any remaining monomer residues and the associated solvent (usually entrained ethylbenzene and toluene, and also residues of the third solvent according to the invention) and the condensing of the vapor stream produced and, if worthwhile, its reintroduction into the plant, directly or after appropriate recovery procedures.
The invention primarily provides a process of the type mentioned at the outset, in which according to the invention at least one system of circulating solvent is provided, where the rubber is used in the form of a solution in a third solvent which is neither styrene nor any alkylaromatic solvent used and whose boiling point is lower than that of styrene and that of any alkylaromatic solvent used and where, in a (first) system of circulating solvent, the third solvent is removed from at least one of the polymerization reactors by distillation with evaporative cooling and utilizing the heat of polymerization and, if desired after appropriate recovery procedures, i.e. directly or indirectly, is introduced into the rubber preparation process which is immediately upstream, and where, in a manner known per se, the impact-modified polystyrene formed is freed from residual styrene and, where appropriate, from minor components, and the vapor stream formed is condensed and, if desired in a second system of circulating solvent, again after appropriate recovery procedures if required, is likewise reintroduced. The alkylaromatic solvent used is preferably ethylbenzene.
As is known, the rubber which can be used according to the invention must be one which is intrinsically compatible with polystyrene in the manner which is typical of particulate graft rubbers, since no grafting occurs in the anionic polymerization. It may be an SB rubber (styrene/butadiene rubber), a styrene-butadiene block rubber or a mixture of styrene-free or low-styrene butadiene rubber and a styrene-butadiene block rubber prepared batchwise or preferably likewise prepared in a continuous process.
The third solvent according to the invention may be an aliphatic, cycloaliphatic or aromatic hydrocarbon having from 4 to about 8 carbon atoms, or mixtures of these with a boiling point (b.p.1013hP) of below 130xc2x0 C. Examples of suitable solvents are pentane, hexane, heptane, cyclohexane, methylcyclohexane, benzene and toluene. It is important that this third solvent fulfills the usual conditions for use in anionic polymerization, in particular freedom from protic substances and oxygen. It is expedient for it to be distilled before use and dried over alumina or molecular sieve.